Lightning protection systems for wind turbine blades usually comprise lightning receptor arrangements having an external metallic receptor element and an electrically conductive internal block connected to a down conductor of a grounding arrangement of the wind turbine. Once captured lightning currents by the receptor element must be transmitted to the electrically conductive internal block that connect the receptor element to the down conductor.
The evolution in the development of wind turbines towards increased power output had led to larger wind turbines both in tower height and rotor diameter.
As an increasing blade length involves and increased rigidity wind turbine blades incorporating carbon fiber laminates are needed. As carbon fiber laminates are conductive they must be connected in parallel with the down conductor to prevent the generation of internal arcs between the down conductor and the carbon fiber laminates and to avoid direct lightning impacts over them.
WO 2006/051147 A1 describes a lightning protection system comprising means for equipotentializing the carbon fiber laminates with the down conductor including auxiliary cables to connect directly the down conductor with the carbon fiber laminates. These auxiliary cables are connected using a joint screwed to a metal plate in direct contact with the layers of carbon fiber. The electrical connection can be improved by adding conductive resins to the joint area.
If the wind turbine blade has, for example, one carbon fiber laminate, the lightning protection system is converted into a two-branched circuit in parallel: one branch formed by the down conductor, of low resistance and high inductance, and the other branch formed by the carbon fiber laminate, having high resistance and low inductance. When lightning strikes on one receptor element, the lightning protection system must evacuate the current, whose waveform is characterized by having a first phase in which the current rises steeply, followed by a second phase where the current drops slowly. When this current is injected into the circuit formed by the carbon fiber laminate connected in parallel to the down conductor, the current is distributed as follows:                During the steep rise phase, most of the current is transmitted by the conductor with less inductance (carbon fiber laminate).        During the gradual drop phase, most of the current is transmitted by the conductor with less resistance (down conductor).        
With the current distribution described above, the carbon fiber laminate undergoes a large current peak at the beginning of the discharge. On the other side as the size of the blades increases, the inductance of the carbon fiber laminates (wider and thicker) decreases causing that the fraction of the current conducted by the carbon fiber laminates increases raising a problem as the carbon fiber laminates contain resins that degenerate at temperatures between 100° C. and 200° C.).
To solve this problem, ES 2 396, 839 A1 discloses the use of a high-inductance device placed in the connection between a carbon fiber laminate and a down conductor to reduce the passage of current through the carbon fiber laminate and favour its conduction through the down conductor.
A problem of lightning protection system of wind turbine blades with carbon fiber laminates is that local lightning current injection to carbon laminates may not be properly distributed producing damage in the injection area. This is caused by the transient nature of the lightning strike together with differences in material conductivities which reduce the effective lightning current injection area of the connection.
Another problem is that the lightning current and voltage distribution between the down conductors and the carbon fiber laminates may not be balanced due to variations of the real values of the impedance of the carbon fiber laminates used in a wind turbine blade with respect to their expected values when the lightning protection system is designed.
This invention is directed to the solution of these problems.